In a conventional power supply device of an electronic control unit, which is mounted in a vehicle, for example, it is necessary to supply power to a microcomputer (hereinafter referred to as computer) operable even in an off-period of a main power supply switch, which is turned on and off in conjunction with an ignition switch. For this reason, the power supply device uses a power supply circuit of small current consumption in many cases. As the power supply circuit of such small current consumption, a linear regulator, which uses N-channel type MOS transistors as switching components for voltage control, is typical.